1. Field of the Invention
The present invention relates to the deposition of a droplet-free nitride wear-resistant coating containing metal components such as Ti, Cr, Al, Si which can be applied, for example, to cutting tools and wear parts.
2. Discussion of the Background
Wear resistant coatings are layers of hard material with thicknesses of approximately 1 to 10 μm, deposited on the tool surface by cathodic arc-evaporation, which is a PVD method (physical vapor deposition).
Cathodic arc-coating methods have already been used for many years for the deposition of wear resistant layers on chip-removing tools. At the Gühring group (G-ELIT Co. Berlin), such a method was developed in recent years by the Coating R&D Department and then used successfully in production and coating service, and was protected under EP 0885981A2. This method is used at Gühring to manufacture TiN and TiAlN coatings, TiAlN/TiN multi-layer coatings and TiAlCrN coatings.
The advantages of the arc-coating technique lie in a very high deposition rate and very dense coating structure, which are achieved by the high energy or momentum input by the metal ions (Ti+, Ti++, Al+, Al++). Approximately 80% of the metal particles vaporized by the cathodic arc are subsequently ionized in the plasma and accelerated as positively charged particles to the tools. Together with the nitrogen atoms from the plasma, this results in growth of a metal nitride coating.
During the coating process, metal microparticles (0.5 to 5 μm) are also incorporated into the coatings. These originate from vaporized material (titanium or titanium/aluminum) of the arc cathode. During the vaporization process, small zones of μm size are locally melted by the cathodic arc as it wanders over the cathode. As a result, the metal droplets are spattered toward the tools to be coated. These “droplets” increase the roughness of the tool and thus hinder chip flow during drilling. For deep hole drilling larger than 3 times the drill diameter, the danger of tool breakage during the first holes is very great. In addition, clogging of the chips in the slot can occur.
Cathodic arc evaporation is a widely employed technique for using dense and highly ionized gas discharges for deposition of hard, wear-resistant PVD coatings. As a result of the high degree of ionization, the high deposition rate and relatively low costs of the electrical power supply units, the cathodic arc-coating method has been used successfully by the manufacturers of PVD coatings, especially in the area of wear resistant coatings for HSS (High Speed Steel) and hard-metal cutting tools. A major disadvantage of the method is the emission of metal microparticles, known as droplets, which during the coating process are transferred from the cathode material into the growing coatings, where they form defects.
While the presence of the droplets prevents application of the arc-vaporization process in the fields of optics, electronics and corrosion protection, the effect of the droplets in PVD coatings for cutting tools has heretofore been tolerated as less critical. In the future, however, smooth surfaces of coated hard-metal tools will become increasingly important in meeting the stringent requirements of tool applications in aerospace, in the automobile industry and in mechanical engineering and in preventing disastrous failure of precision tools during service.